The Qiantang River runs for 459 km through Zhejiang, China, and flowing into the East China Sea via Hangzhou Bay. From Aug 15th to 21st in the Chinese Lunar Calendar, travelers come to Hangzhou Bay to see the largest tidal bore in the world, which can reach up to 9 meters in height and travel at up to 40 km/h.
There are several reasons contributing to the spectacular tidal bore. First of all, from Aug 16th to 18th in the Chinese Lunar Calendar, the Sun, the Earth and the Moon are approximately in the same line. The gravitational forces of both the Sun and the Moon work together to create the greatest difference between high tide and low tide, which is called the spring tide.
In addition, the Qiantang River has a horn-shaped estuary, so the river surface suddenly becomes narrow when the water reaches the Hangzhou Bay. The abundance of sediment on the river bed slow down tides that come first, forcing waves behind to drive on those ahead. The prevailing southwest wind in coastal areas in China also helps speed up the tidal bores.